Ultrasonography
Ultrasonography, also known as sonography, is a technique used in medical imaging in which high-frequency sound waves (typically between 1 and 20 MHz) are reflected off internal organs and the echo pattern is converted into a picture of the structures beneath the transducer. Because ultrasound images are captured in real-time, they can show the structure and movement of the body's internal organs, as well as blood flowing through blood vessels.
Ultrasound examinations can help to diagnose a variety of conditions and to assess organ damage following illness. Ultrasound is used to help physicians diagnose symptoms such as pain, swelling and infection. Ultrasound is a useful way of examining many of the body's internal organs and conditions, including but not limited to the: heart and blood vessels, including the abdominal aorta and its major branches (for example, for abdominal aortic aneurism); liver; gallbladder; spleen; pancreas; kidneys; bladder; uterus, ovaries, and unborn child (fetus) in pregnant patients; ectopic pregnancy; eyes; thyroid and parathyroid glands; scrotum (testicles); and breast. Ultrasound is also used to diagnose a variety of heart conditions and to assess damage after a heart attack or other illness.
In addition, ultrasound is increasingly used to guide medical procedures such as those involving needle puncture. Examples include, but are not limited to; needle delivery of anesthesia; placement of central venous catheters; placement of pulmonary artery catheters; needle biopsy and fine needle aspiration; amniocentesis; femoral catheter placement; and, egg harvesting. In these and other applications, a sterile coupling gel is often used. The ultrasound transducing surface is coated with a sterile or non-sterile gel, and then the transducer may be placed in a sterile fragile sheath. The outer surface of the sheath, or the surface of the patient's skin, is then coated with sterile ultrasound gel.
Conventional methods for targeted delivery of local anesthesia have been utilized with varying success for decades. A traditional method includes assessing needle location using the tactile feedback (clicks, pops) that the needle generates as it penetrates tissue adjacent to the desired nerve site. Another method attempts to correctly locate the anesthesia needle using paresthesia, the abnormal neurological sensations that results when the needle touches the intended nerve.
A slightly more advanced method to guide anesthesia delivery which has largely supplanted the older methods of clicks, pops or paresthesia is that of nerve stimulation. In this method, an insulated needle is attached to an electrically charged live wire. As the needle approaches the nerve, the patient will experience an involuntary movement caused by the electrically charged needle stimulating the desired nerve once it is sufficiently close to the nerve.
In addition to being unreliable in correctly identifying the nerve, these conventional procedures are fairly slow and can be unsafe to the patient due to the need for multiple and/or incorrectly placed injections. Ultrasound guided delivery of anesthesia provides a more effective, safer and faster alternative to these conventional approaches.
Central venous catheter line placement has also been traditionally executed using a ‘feel’ approach. Certain anatomical landmarks such as bones are used to identify the location of the jugular vein. However, obesity, vascular disease, hypotension, and many other factors can create a unique set of challenges in correctly identifying the location for even the most experienced healthcare provider. The American College of Emergency Physicians has recognized the importance of this skill by including it in the 2001 policy statement “Use of Ultrasound Imaging by Emergency Physicians” (Ann. Emerg. Med. 2001; 38:469-70), which calls ultrasound-guided central venous access one of the “primary applications for emergency ultrasound.”
In performing needle biopsy, such as breast biopsy, ultrasound guidance has proven quite valuable. After placing an ultrasound probe over the site of the breast lump and using local anesthesia, the radiologist guides a biopsy needle directly into the mass. Tissue specimens are then taken using either an automatic spring-loaded or vacuum-assisted device (VAD). Ultrasound is most often used to guide breast biopsy when a breast abnormality is visible on ultrasound. When it is necessary to do an open surgical biopsy, a guide wire first is passed directly into the mass. This procedure also may be guided by ultrasound.
Other broad applications of sonography include phonophoresis and wound healing. Phonophoresis (also known as sonophoresis or ultrasonophoresis) is the movement of a medication or other substance through the skin by the application of sonic radiation to the medicament placed upon the skin. In wound healing, ultrasound plays a role because it has been well established that ultrasound by itself can speed up the healing process in open wounds.
More recently, the use of high intensity focused ultrasound (HIFU) for therapeutic purposes, as opposed to imaging, has received significant attention in the medical community. HIFU therapy employs ultrasound transducers that are capable of delivering 1,000-10,000 W/cm2 to a focal spot, in contrast to diagnostic imaging ultrasound, where intensity levels are usually below 0.1 W/cm2. A portion of the energy from these high intensity sound waves is transferred to the targeted location as thermal energy. The amount of thermal energy thus transferred can be sufficiently intense to cauterize undesired tissue, or to cause necrosis of undesired tissue (by inducing a temperature rise to beyond 70° C.) without actual physical charring of the tissue. Tissue necrosis can also be achieved by mechanical action alone (i.e., by cavitation that results in mechanical disruption of the tissue structure). Further, where the vascular system supplying blood to an internal structure is targeted, HIFU can be used to induce hemostasis. The focal region of this energy transfer can be tightly controlled so as to obtain necrosis of abnormal or undesired tissue in a small target area without damaging adjoining normal tissue. Thus, deep-seated tumors can be destroyed with HIFU without surgical exposure of the tumor site.
Ultrasound Coupling Gels
Sound waves are poorly transmitted by air and thus require a coupling mechanism for proper transmission. This coupling mechanism is commonly a viscous fluid or gel which, due to its physical and acoustic properties, acts to displace air, fill contours between the piezoelectric transducer and the body, and enable successful transfer of the acoustic energy. Many ultrasound coupling gels exist in the market place in both sterile and non-sterile forms. Sterile ultrasound gels include Sterile Aquasonic® 100 (Parker Labs, Inc., Orange, N.J., 07050), Ultra/Phonic™ (Pharmaceutical Innovations, Newark, N.J.), UltraBio Sterile (Sonotech, Bellingham, Wash., 98225) and Sonogel-Sterile (Sonogel Vertriebs GmbH, D-65520 Bad Camberg). Sterile ultrasound gels are typically provided in single-use individually wrapped sterile foil pouches of 20 g each. The UltraBio product (U.S. Pat. No. 6,866,630 to Larson et al.) describes an in vivo biocompatible, bioeliminating sterile diagnostic ultrasound imaging couplant and lubricant.
Some ultrasound applications, for example fetal ultrasound, desire the ability to continuously move and reposition the ultrasound probe in order to gain multiple images at multiple angles of multiple sites. Other ultrasound applications, such as but not limited to the ultrasound guided procedures described above, desire the ability to move the ultrasound transducer until an optimal position is located, and then desire the probe to remain stable in this position until the user intentionally adjusts the position of the probe. In these applications, the ultrasound probe is commonly held in one hand of the caregiver while the procedure requiring guidance is performed with the other hand. Current ultrasound coupling gels generally exhibit a slippery or low friction state and thus leave the probe susceptible to unwanted movement, potentially leading to loss of visualization of the target site, the need to relocate the site, misguided or repeat punctures and an overall decrease in safety, effectiveness and procedure efficiency.
Efforts to utilize adhesive or bioadhesive coupling agents have been disclosed. U.S. Pat. No. 5,522,878 to Montecalvo et al. describes a solid, multipurpose, flexible, ultrasonic, biomedical couplant hydrogel in sheet form to facilitate transfer of ultrasound energy to and from a patient. Also described is a method of attaching the sheet to skin to hold the couplant gel in place during an exam, which constitutes a band of pressure sensitive adhesive bonded to plastic foam, such as foamed rubber, that is located along the outer perimeter of the sheet. The hydrogel sheet described is not adhesive in and of itself, but depends on sufficient perspiration to make the gel somewhat tacky. The adhesive border, so described, is not acoustic self-coupling, therefore restricting ultrasound scanning to areas exclusive of those covered with adhesive covered foam. The level of adhesion of the hydrogel sheet is fixed and the tacky surface is only between the skin and the sheet. It is meant to cling to the skin while the transducer moves freely on top of it. It is not intended to aid in the maintenance of the probe's final position, but rather to improve ease of handling by being easily applied to and removed from the body.
U.S. Pat. No. 6,719,699 to Smith describes adhesive hydrogel films or sheets as acoustic coupling media attachable to the active face (transducer) of ultrasound instruments (such as probes or scanheads) and to the inner face of latex, polyurethane or other polymeric probe covers; thereby, enabling the transfer of acoustic energy between an ultrasound probe and an object of interest when used in conjunction with a gel or liquid ultrasound couplant on the skin surface. The adhesive hydrogel comprises acoustic transmission media and is adhesive on both sides of the film. Such adhesive hydrogels films are so comprised as to render desirable levels of acoustic transmission with acceptable low levels of acoustic artifacts, distortion and attenuation. The invention of U.S. Pat. No. 6,719,699 allows for an adhesion between the probe surface and the inner face of a probe cover. The invention does not aid in the effort of helping to secure or fix the probe in an intended desired position. Nor does the invention describe a gel which varies in adhesion and viscosity over time.
U.S. Pat. No. 5,394,877 to Orr et al. describes a contact medium structure attachable to externally applied medical diagnostic devices for providing self-adherence of a medical device to the skin of a patient thereby eliminating the need for retaining belts or similar means. A contact medium is described that is inherently adhesive, hydrophilic, skin compatible, ultrasonic compatible and pressure sensitive to facilitate self-adhesion of the medical device to the patient's skin. The device of Orr et al. necessitates the use of a flexible support element which must be manually set in place to fix the ultrasound probe in its desired position. The inherently adhesive contact medium has a fixed adhesion. It does not allow for easy positioning of the transducer followed by a natural and automatic inherent increase in adhesion and viscosity to assist in holding the transducer in the desired position. Because the invention utilizes the flexible support element which holds a mesh-reinforced hydrogel film in place, it is not conducive to ultrasound guided procedures such as needle guided procedures as previously discussed.
U.S. Pat. No. 5,070,888 to Hon et al. details the use of a strong adhesive on the abdomen of a patient that forms a solid bond with the skin in order to secure the transducer to the patient. U.S. Pat. No. 4,920,966 to Hon et al. describes an adhesive layer applied to the surface of a disc-shaped transducer base in contact with the skin. Such an adhesive in these patents is sufficiently strong to maintain the transducer in place on the patient without the use of a belt. However, such a system is difficult to remove because the adhesive would bond to the skin of the patient and require the use of solvents for the removal of the transducer from the patient. The inherently adhesive contact medium has a fixed adhesion. It does not allow for easy positioning of the transducer followed by a natural and automatic inherent increase in adhesion and viscosity to assist in holding the transducer in the desired position.
U.S. Pat. No. 6,048,323 to Hon et al. describes the use of a hydrogel layer present on the lower surface of a fastening pads for attachment to the patient. The hydrogel is a mild adhesive which is sufficiently strong to provide the necessary fixation forces to fix the transducer support plate on the patient, but does not form a strong bond with the skin of the patient. The hydrogel is easily removed from the skin of the patient without the use of solvents. This hydrogel requires the use of an additional fastening pad, and does not experience the increase in adhesion over time that first allows positioning before aiding in anchoring the transducer in the desired position. It is also not suitable for guided injection procedures due to the use of the fastening pad.
In light of the aforementioned problems with current techniques, it would be advantageous to have an ultrasound coupling gel which initially allows for easy movement and positioning of the ultrasound transducer, and then inherently increases in viscosity and adhesion over time, assisting in fixing the probe to the skin in the desired position, all the while providing for consistent contact with the patient's skin to allow for proper transmission of the wave signal. Additionally, it would be advantageous to have the aforementioned product which allows for unimpeded ultrasound guided procedures such as needle injection, and the ability to intentionally remove or reposition the transducer if desired even after ultimate adhesion has been achieved.